1. Field of the Invention
This invention relates to a photovoltaic element using microcrystalline silicon germanium (xcexcc-SiGe) as a photoactive layer.
2. Description of Prior Art
A photovoltaic device mainly containing amorphous silicon (referred as a-Si herein after) formed by photo CVD and glow discharge decomposition of material gas is featured by that it can be a thin film and is easy to expand its dimension, and has been expected to be manufactured at low cost.
A pin-type a-Si photovoltaic device having pin junction is a general photovoltaic device. FIG. 9 illustrates a structure of such the photovoltaic device. The device is formed by laminating a transparent electrode 22, a p-type a-Si layer 23, an intrinsic (i) type a-Si layer 24, an n-type a-Si layer 25, and a metal electrode 26 on a glass substrate 21 in this order. When light is incident through the glass substrate 21, the photovoltaic device generates photovoltaic power.
Regarding the a-Si photovoltaic device, it is known that light degradation occurs after light is irradiated. To solve this problem, a photovoltaic device using microcrystalline silicon, material which is thin and has high stability to light irradiation, as a photoactive layer has been proposed. The microcrystalline silicon is a thin film having mixture of a microcrystalline silicon (Si) phase and an amorphous silicon (a-Si) phase.
As described above, the microcrystalline silicon (Si) has been paid attention as a technique for overcoming the problem of light degradation in the amorphous silicon (a-Si) based semiconductor layer. However, a light absorption coefficient of the microcrystalline silicon is less as compared with the amorphous silicon, and a thickness of 2 xcexcm or more is required for using as a photoactive layer. Therefore, a very high forming speed is required when considering productivity of solar cells. With the present state, however, such forming speed can not be achieved while maintaining good characteristics.
To solve the problem, this invention uses microcrystalline silicon germanium (SiGe) having a greater light absorption coefficient than the microcrystalline silicon and makes a thinner photoactive layer. To overcome the problem, the following factor must be satisfied.
In order to make the photoactive layer 1 xcexcm or less in thickness, a light absorption coefficient of the microcrystalline silicon germanium must be at least three times as great as of the microcrystalline silicon. To achieve such the light absorption coefficient, a composition ratio of germanium (Ge) in the microcrystalline silicon germanium (SiGe) should be 20 at. % or more.
This invention is made to provide a photovoltaic device using a microcrystalline silicon based semiconductor thin layer having a small thickness as a photoactive layer.
A photovoltaic device of this invention comprises a supporting substrate, a first electrode formed on the supporting substrate, a semiconductor layer including a photoactive layer and formed on the first electrode, and a second electrode formed on the semiconductor layer. The photoactive layer is a microcrystalline silicon germanium layer which a Ge composition ratio is 20-40 at. %, a signal intensity of a germanium-to-germanium bond is 30-60% of a signal intensity of a silicon-to-silicon bond observed by the Raman spectroscopy, and a signal intensity of a silicon-to-germanium bond is between the above two signal intensities, and a thickness of the photoactive layer is 1 xcexcm or less.
The microcrystalline silicon germanium layer is satisfied that the signal intensity of the germanium-to-germanium bond is 35-55% of the signal intensity of the silicon-to-silicon bond observed by the Raman spectroscopy.
The Ge composition ratio of the microcrystalline silicon germanium layer is 20-40%.
A photovoltaic device of this invention comprises a supporting substrate, a first electrode formed on the supporting substrate, a semiconductor layer including a photoactive layer and formed on the first electrode, a second electrode formed on the semiconductor layer. The photoactive layer is a microcrystalline silicon germanium layer which a Ge composition ratio is 20-40 at. %, a crystal size is 5-300 xc3x85, and a thickness of the photoactive layer is 1 xcexcm or less.
The crystal size of the microcrystalline silicon germanium layer is 10-200 xc3x85.
The photovoltaic device with the above structure using the thin microcrystalline silicon germanium as the photoactive layer can provide good conversion efficiency.